Process for preparing high viscosity linear condensation polyesters from partially polymerized glycol terephthalates



Oct. 8, 1968 H. H. HEIGHTON ETAL 3,405,098

PROCESS FOR PREPARING HIGH VISCOSITY LINEAR CONDENSATION POLYESTERS FROM PARTIALLY POLYMERIZED ALYCOL TEREPH'IHALATES Filed Oct. 29, 1965 2 Sheets-Sheet 1 AIEIORPIIOUS PREPOLYNER CHIPS 0F POLYETHYLEIIE TEREPHTIIALATE HAVING MI INTRINSIC VISCOSITY PREHEAT AT law-20090.10 PARTIALLY CRYSTALLIZE ms PREPOLYIIER CRIIIID PREPOLYIIER INTO PARTICLES PASSING 20-IIESH SCREEN FLUlDIZED-BED SOLID PHASE POLYMERIZATION AT 2oo-23sc. TO AT LEAST M INTRINSIC VISCOSITY INVENTORS HAROLD H. HEIGHTON ELMER E. MOST, JR.

ATTORNEY Oct. 8, 1968 H. H. HEIGHTON ETAL 3,405,093

PROCESS FOR PREPARING HIGH VISCOSITY LINEAR CONDENSATION POLYESTERS FROM PARTIALLY POLYMERIZED ALYCOL TEREPHTHALATES Filed Oct. 29, 1965 2 Sheets-Sheet 2 ATTORNEY United States Patent 3,405,098 PROCESS FOR PREPARING HIGH VISCOS- ITY LINEAR CONDENSATION POLY- ESTERS FROM PARTIALLY POLYMER- IZED GLYCOL TEREPHTHALATES Harold H. Heighton and Elmer E. Most, Jr., Kinston,

N.C., assignors to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware Continuation-impart of application Ser. No. 417,004, Dec. 9, 1964. This application Oct. 29, 1965, Ser. No. 513,616

6 Claims. (Cl. 260-75) This is a continuation-in-part of our application Ser. No. 417,004, filed Dec. 9, 1964, now abandoned. The invention relates to an improved process for preparing high viscosity linear condensation polyesters from partially polymerized glycol terephthalates, and is more particularly concerned with fluidized-bed solid phase polymerization of polyethylene terephthalate.

Polyethylene terephthalate of high molecular weight, having an intrinsic viscosity of 0.8 and higher, is desirable for preparing industrial yarns wherein the increased molecular weight of the polymer results in yarns of increased strength. Such high viscosity polymer is not readily prepared, if at all, by melt-polymerization techniques because of the difiiculty of processing highly viscous masses and because attempts to increase the temperature to reduce the viscosity results in polymer degradation.

. Solid phase polymerization, wherein a low molecular weight prepolymer is further polymerized at temperatures below the melting point, is effective for producing polymer of high intrinsic viscosity without polymer degradation. In order to provide reasonable rates of polymerization, it is necessary to provide for uniform heating and rapid removal of condensation by-products. This can be accomplished by a fluidized-bed solid phase polymerization, in which the material is polymerized in the form of particles of sufficient fineness to be agitated in a gas stream and be handled like a liquid. A commercial process for solid phase polymerization of polyamides in this manner is disclosed by Monroe in US. Patent No. 3,031,- 433, issued Apr. 24, 1962.

Attempts to use fluidized-bed polymerization to prepared polyethylene terephthalate on a commercial scale have encountered serious difficulties. Polyester of less than 0.8 intrinsic viscosity is conventionally prepared by melt polymerization of the monomer components and then rapidly quenching the molten polyester, as by casting on a water-cooled surface. This gives an amorphous product. Amorphous polyethylene terephthalate is quite tough and difficult to grind into particles with conventional equipment. It also has a low melting point, e.g., 120 C. for polyester of 0.4 intrinsic viscosity, which causes agglomeration of particles during grinding and in the fluidized-bed.

A highly crystalline product can be obtained by slow cooling of the molten polyester, e.g., by casting the polyester as a thick sheet which is allowed to cool slowly. This crystalline polyester can be ground to suitable particle sizes without difiiculty in commonly available grinding machines. Furthermore, the crystalline melting point is about 265 C. and polymerization temperatures up to 235 C. can be used without agglomeration of particles. However, it is found that the rate of polymerization is excessively slow and that an intrinsic viscosity of 0.8 is not reached, even after many hours of heating at temperatures below 235 C.

The present invention provides an improved process suitable for preparing polyethylene terephthalate of unusually high intrinsic viscosity. It also provides for solid phase polymerization at rapid rates without agglomera- 3,405,098 Patented Oct. 8, 1968 tion of particles or degradation of polymer. Other advantages of the invention will become apparent from the specification and drawings.

In accordance with this invention polyethylene terephthalate having an intrinsic viscosity of 0.20 to 0.65 is first prepared by polymerizing monomer components in a conventional manner to give an amorphous melt of prepolymer. The melt is quenched rapidly to form a substantially amorphous solid prepolymer and this is broken into chips. The prepolymer chips are heated at 150 C. to 200 C. until the polyester is partially crystallized. Preferably the polyester is heated at about 160 C. for at least 10 minutes, or until polyethylene terephthalate has a density within the range of 1.39 to 1.40. The partially crystallized polyester is next ground into particles passing a 20-mesh screen, with less than 10% fine enough to pass through a ZOO-mesh screen. Preferably the particles pass through a 40-mesh screen and are retained on a mesh screen. The polyester is then polymerized to an intrinsic viscosity of at least 0.8 by heating the particles at 200 C. to 235 C. in a fluidized bed. Preferably this solid phase polymerization is conduced at 215 C. to 225 C.

In the drawings, which illustrate the invention:

FIGURE 1 is a flow diagram to illustrate the process of this invention,

FIGURE 2 is a plot of data from Example II, showing the change of intrinsic viscosity with time of polymerization for propolymer samples which have previously been crystallized at diiferent preheat temperatures, and

FIGURE 3 is a plot of polyester density vs. time of preheat for different preheat temperatures.

The process of FIGURE 1 may be carried out by charging the amorphous prepolymer chips to a conventional type of preheater wherein the chips are heated to a temperature of ISO-200 C. to partially crystallize the prepolymer. After crystallization, the chips may be ground with conventional equipment to the desired particle size and charged to a fluidized-bed polymerizer, such as one of the type disclosed in Monroe US. Patent No. 3,031,433.

The polyethylene terephthalate prepolymer used in the practice of this invention may be prepared in any suitable manner known to the art. The polymerization is carried out to produce a prepolymer having an intrinsic viscosity between 0.20 and 0.65, preferably about 0.40. The prepolymer is broken into chips that will pass through 0.125- inch openings of a screen. The chips are ground into particles after they have been heated to induce partial crystallization. Prepolymer having an intrinsic viscosity less than about 0.20 results in the formation of excessive fines, for example, as much as 50% or more will pass through a standard ZOO-mesh screen when subsequently ground in conventional grinding equipment. Above an intrinsic viscosity of about 0.65, the prepolymer becomes very diflicult to grind and, further, requires longer heating periods to obtain sufiicient crystallinity to prevent or minimize fusing of the particles during the polymerization. The preferred intrinsic viscosity is about 0.4, which represents a compromise between those conditions in that it allows the preparation of a crystalline prepolymer in a reasonable period of time and the resulting material can be readily ground to the desired particle size with less than 10% fines.

Amorphous polyethylene terephthalate having an intrinsic viscosity of 0.4 has a melting point of C. Since solid-phase polymerizations must be carried out at temperatures of at least about 200 C. to provide suitable rates of polymerization, and since at these temperatures the amorphous prepolymer melts before it can crystallize, the disadvantage of an amorphous prepolymer is apparent.

a Polyethylene terephthalate crystals have a melting point of about 265 C. and prepolymers having a relatively high percent of crystallinity are quite suitable. The crystallinity developed in an amorphous prepolymer by heating it at a temperature of about 160 C. for at least minutes is sufficient to permit its use for solid-phase polymerization at temperatures up to about 235 C. without its becoming tacky or otherwise interfering with the efficiency of the operation. Further, the crystallinity developed at 160 C. is such that grinding is readily accomplished. With respect to the ease of grinding, no significant difference is noted between samples heated at 150 C. and those heated at temperatures of 200 C. and higher.

The invention is most advantageously used in a continuous, fluidized-bed, solid-phase polymerization process. The temperature at which the preheat step is carried out lies between 150 and 200 C. A temperature of about 160 C. is preferred since it leads to optimum results. A preheat temperature of about 160 C. gives the maximum rate of polymerization and the ground prepolymer can be used in fluidized-bed polymerization at temperatures up to about 235 C. without becoming tacky and thus impairing the fluidization of the bed. Preheat temperatures as low as 150 C. may be used to advantage but at some sacrifice in bed fiuidization unless somewhat lower bed temperatures are used. Preheat temperatures up to 200 C. may be used to prepare polymer having an intrinsic viscosity of about 0.8 at which point the increase in polymerization time becomes excessive for the higher temperatures. Apparently, preheat temperatures above 160 C. decrease polymer chain end mobility and glycol diffusion rate, thus retarding polymerization rates. For intrinsic viscosities desired for industrial yarns such as those above about 0.8, a preheat temperature of about 160 C. is especially preferred. For example, a prepolymer sample preheated at 160 C. for minutes will reach an intrinsic viscosity of 0.8 at a polymerization temperature of 217 C. in about one-half the time required for a prepolymer sample preheated at 217 C. for 15 minutes. At intrinsic viscosities greater than 0.9, the difference in polymerization times to reach a given level becomes even more pronounced.

In preheating the prepolymer chips, it is desirable, of course, to bring the prepolymer to the desired temperature as rapidly as possible without, however, heating so rapidly that the prepolymer fuses into a mass before it begins to crystallize. The melting point rises rapidly as crystallization occurs and, after initial crystallization has occurred, the prepolymer can then be heated to the final crystallization temperature, in the range of 150 to 200 0, without fusion occurring. Crystallization to a density of 1.39 to 1.40 occurs quickly at the temperatures indicated in FIG- URE 3, a few minutes being sufiicient. The time required will, of course, vary with the bulk of the prepolymer being heated and the heating equipment. Suitable conditions to prevent particle fusing will be apparent to those skilled in the art. No significant change occurs in the intrinsic viscosity during the preheat period.

As shown by FIGURE 3, a preheat temperature of 160 C. offers the additional advantage of reaching a constant level of crystallinity, as reflected by density measurements. Such a characteristic is especially advantageous in a continuous process wherein it is relatively easy to control the uniformity of the product, particularly in the event of delays resulting from difficulties occurring in a subsequent stage of the process.

After the desired preheating, the prepolymer is ground to the desired particle size. The particle size may be between and 200 mesh. As is well known, the smaller particle sizes result in faster polymerization rates. However, grinding the prepolymer to too small a particle size results in a high proportion of fines which are objectionable in a fluidized-bed process. Accordingly, it is preferred that the particle sizes be between 40 and 100, i.e., th ground particles will pass a 40-mesh screen but will be substantially all retained by a 100-mesh screen.

The ground prepolymer is then polymerized to the desired intrinsic viscosity by well-known, solid-phase polymerization methods at temperatures above 200 C. The preferred polymerization temperatures lie between 215 and 225 C. Below 215 C. the rates are so slow that the process may be uneconomical and above 225 C. fluidization of the particles may be difficult to maintain in a continuous process because of agglomeration. Above 235 C., agglomeration occurs no matter how crystalline the prepolymer may be.

A further advantage realized by solid-phase polymerization in accordance with this invention is production of polymer containing very few free carboxyl groups. The unexpected susceptibility of polyester fibers to heat degradation in contact with-rubber has been described by Daniels in U.S. Patent No. 3,051,212, dated Aug. 28, 1962 which teaches the importance of a low concentration of free carboxyl groups in the polymer in controlling such degradation. The high molecular weight polyester prepared by the present invention is found to have less than 10 free carboxyl equivalents per million grams, whereas a high molecular weight polyester prepared by conventional and well-known melt polymerization is found to have greater than 20 carboxyl eq./1O g. of polyester.

Intrinsic viscosity is determined at 25 C. in an Ostwald viscometer using a solution of 0.32 gram of the polyethylene terephthalate in ml. of a solution of 75% methylene chloride and 25% trifluoroacetic acid. The values reported are accurate to within 10.02.

As is well known (see Crystallization of Polymers by Leo Mandelkern, pages 215-218, McGraw-Hill Book Co., 1964), as the degree of crystallinity of a polymer increases, the polymer becomes increasingly dense. The density values given herein were measured in a density gradient tube against standards established with carbon tetrachloride as the heavy liquid and n-heptane as the light liquid.

EXAMPLE I This example describes the preparation of a prepolymer useful in the practice of this invention. Bis-,B-hydroxyethyl terephthalate is prepared continuously from ethylene glycol and dimethyl terephthalate by the method of Vodonic, U.S. Patent No. 2,829,153, dated Apr. 1, 1958, using a catalyst comprising manganous acetate and antimony oxide. This monomer is then supplied continuously to a polymerization system where the temperature is raised and the pressure reduced to produce polyethylene terephthalate prepolymer. The final stages of polymerization are carried out at a temperature of about 270 C. and a pressure of about 4 mm. of mercury, using the finishing apparatus described by Willey in U.S. Patent No. 3,046,099, dated July 24, 1962, the presssure being controlled to give the desired intrinsic viscosity. The outlet end of the finisher is fitted with a screw pump similar to that described by Bendett in U.S. Patent No. 2,805,627, dated Sept. 10, 1957, to facilitate the transfer of prepolymer directly to a casting machine. Molten prepolymer is extruded through a 3-inch slot onto a chilled casting wheel to obtain a glassy amorphous ribbon which is then cut to pass 0.125-inch openings of a screen to give a supply of amorphous prepolymer chips. The prepolymer has an intrinsic viscosity of 0.39. I

The amorphous prepolymer is then ground to a particle size of 60-80 mesh in a rotary mill. The amorphous prepolymer is ground, contrary to the preferred sequence, prior to preheating to give a uniform supply for test purposes and to eliminate the inconvenience of grinding the small amounts of prepolymer used in the experiments described in Examples II-IV.

EXAMPLE II Sufficient prepolymer of Example I to give six l-gram samples is heated, under dry nitrogen, at C. for 15 minutes in a glass tube to cause crystallization and is then cooled.

After preheating, one-gram samples of the prepolymer vare plaeed in the bottoms of 30-millimeter diameter glass polymer-tubes, Six tubes are used and they are connected to a common manifold-and vacuum maintained "Samples of the amorphous prepolymer of Example I are preheated, in Runs 6 and 7 at temperatures of 190 C. and 215 C., respectively, for 15 minutes as described in Example II. The prepolymer so treated is then polymerized by an oil diifusion pump. The tubes are immersed in a 5 using the procedure of Example II except that a polymcommon, stirred, silicone oil bath. maintained at a temerization temperature of 225 C. and a pressure of 0.005 perature of about 218 C., at which temperature the 0.006 millimeter of mercury is maintained during the Center of the Polymer bed fcachfis thg Polymerization polymerizations. The results are given in Table 2. temperature within 5 minutes. At one-hour intervals, a TABLE 2 tube is removed from the bath and allowed to cool to room temperature without breaking vacuum. The sample Tfr tis gh giggfig is then removed from the tube and its intrinsic viscosity determined. The conditions and results obtained are shown 6 190 g 8- $3 in Table 1 and identified as Run 1. 2 0. 94 In separate runs, the above procedure is repeated except i for the preheat temperature. In Runs 2, 3, 4, and 5 the 5 1 14 preheat temperature is 190 0., 200 0, 218 c. and 215 g 32;, 235 C., respectively. I g 1 5; TABLE L-POLYMERIZATION CONDITIONS 3 0:82 Preheat Time Temp. Press. Intrinsic g Temp.,0. (hours) C.) (mm.Hg) Viscosity 6 1.02 Run 160 1 i "6:00?" gi In this example, a preheat temperature of 190 C. gig 8:882 3:3 results in a polymer having an intrinsic viscosity of 0.8 4 218 0.000 1.00 in about 1.1 hours, and a value of 1.0 in about 2.6 hours 2 gig 3:83? while the higher temperature requires about twice as long to reach the same viscosity levels. The economic ad- 190 "me 35g"- 81? vantage of a controlled preheat step is readily apparent.

2 iii 8:88? 8:3? EXAMPLE v 6 216 M05 02 Amorphous prepolymer prepared as in Example I, and g-g having an intrinsic viscosity of approximately 0.40 is 2 216 01005 0:74 crystallized by heating at a temperature of 160 C. for 2 2 a: $881 8-23 hours before grinding to a particle size of 6080 mesh in a 5 216 01004 0:93 rotor and stator-type grinder. The cold, ground prepoly- 6 216 M04 mer (36.29 kilograms) is heated 12 hours at 160 C., Run 4 21s 0 0.40 charged directly to a fluidized bed polymerizer, and the g 8:888 gzgg batch temperature is raised to 200 C. during 1.25 a 218 0.006 0.68 hours. The temperature is then raised to 220 C. during 2 gig g-gg; 8'3; the next three hours and held at this temperature for 18 6 218 01007 0: 92 hours and 35 minutes. Dry, hot inert gas is passed through Run 5 235 0 O 40 the bed to remove volatiles, supply heat and to maintain 1 'iis "(rene 0:40 fluidization. A sample removed at the end of polymerizagig 8:382 811% tion is found to have an intrinsic viscosity of 1.22. 4 218 0.005 0.60 The process of this invention is also suitable for the g 313%; gig; preparation of high viscosity poly(trimethylene terephthalate) as illustrated in the following example: The data from this example are plotted in .FIGURE 2. The disadvantage of exposing the prepolymer to high EXAMPLE VI temperatures. such as Occurs in a slow cooling of the melt. Poly(trimethylene terephthalate) flake with intrinsic is shown y n 5 wherein an intrinsic viscosity of viscosity of 0.59 and density of 1.33 g./cm. is divided s n achieved even after 6 hours of Polymerization into batches of about 65 pounds each and preheated at EXAMPLE HI 160 for 2 hours to alfect crystallization as indicated by an increase in polymer density to 1.34 g./cm. Crys- Thls example Shows afided advantage of a tallized flake is out twice through a mill using a screen preheaf temperatur? that Permits the with inch diameter holes. The finer flake is preheated ready preparation of vnlfom} Product In terms of again for 6 hours at 165 0. before charging. After crystallinity as reflected by density measurements. Samcharging to the fluidized polymerizer, the flake is P the amorphous prepolymer of Example I are heated to 210 C. with inert gas for 8 hours. At the end crystalllled at temperatures and of this polymerization cycle, the intrinsic viscosity aver- 235 C. for periods of time up to 7.5 hours. At mterages mediate times, the densities of the prepolymer samples Since many diherem embodiments of the invention may are determined by removing portions and evaluating them be made without departing from the Shh-it and scope in a y g f tube as prevlously descl'lbed- The thereof, it is to be understood that the invention is not Sults shown In FIGURE AS can be Seen from limited by the specific illustrations except to the extent graph, the sample preheated at 160 C. reaches a max1- d fin d m the following claims. mum crystallinity in 15 minutes and the value remains We claim; PP for as long as At 7 the crystal 1. In the preparation of linear condensation polyesters 0 approaches 'maxlmuffl 15 mlnutes but slowly wherein glycol terephthalate is partially polymerized to f f f Y P P At crystal form an amorphous melt of polyester, the melt is solidified lin'lty 18 still increasing after more than five hours of and the polymerization is continued in the solid phase Preheat the process for preparing polyester of unusually high in- EXAMPLE Iv trinsic viscosity which comprises the combination of steps: This example shows the advantage of this invention at (a) quenching the melt rapidly to form a substantially an increased polymerization temperature. amorphous solid polyester, the molten polyester be- 7 ing partially polymerized to an intrinsic viscosity of 0.20 to 0.65 when quenched,

(b) heating the solid polyester at 150 to 200 C. to

form a partially crystallized polyester,

(c) grinding the partially crystallized polyester into particles passing a 20-mesh screen, with less than 10% fine enough to pass through a ZOO-mesh screen, and

(d) polymerizing the ground polyester at 200 to 235 C. in a fluidized-bed solid-phase polymerization to an intrinsic viscosity of at least 0.8.

2. A process as'defined in claim 1 wherein the polyester is polyethylene terephthalate and is quenched in step (a) at an intrinsic viscosity of about 0.4.

3. A process as defined in claim 1 wherein the polyester is heated in step (b) at about 160 C. for at least 10 minutes to develop crystallinity.

4. A process as defined in claim 1 wherein the partially crystallized polyester is ground in step (c) to particles which pass through a 40-mesh screen and are retained on a IOO-mesh screen.

References Cited UNlTED STATES PATENTS 3,075,952 1/1963 Coover et al 260-75 3,117,950 l/l964 Kibler et al 260-75 3,305,532 2/1967 Middleburg et al 2607S 3,344,091 9/1967 RuSSin et al 2602.3

WILLIAM H. SHORT, Primary Examiner.

LOUISE P. QUAST, Assistant Examiner. 

1. IN THE PREPARATION OF LINEAR CONDENSATION POLYESTERS WHEREIN GLYCOL TEREPHTHALATE IS PARTIALLY POLYMERIZED TO FORM AN AMORPHOUS MELT OF POLYESTER, THE MELT IS SOLIDIFIED AND THE POLYMERIZATION IS CONTINUED IN THE SOLID PHASE, THE PROCESS FOR PREPARING POLYESTER OF UNUSUALLY HIGH INTRINSIC VISCOSITY WHICH COMPRISES THE COMBINATION OF STEPS: (A) QUENCHING THE MELT RAPIDLY TO FORM A SUBSTANTIALLY AMORPHOUS SOLID POLYESTER, THE MOLTEN POLYESTER BEING PARTIALLY POLYMERIZED TO AN INTRINSIC VISCOSITY OF 0.20 TO 0.65 WHEN QUENCHED, (B) HEATING THE SOLID POLYESTER AT 150* TO 200*C. TO FORM A PARTIALLY CRYSTALLIZED POLYESTER, (C) GRINDING THE PARTIALLY CRYSTALLIZED POLYESTER INTO PARTICLES PASSING A 20-MESH SCREEN, WITH LESS THAN 10% FINE ENOUGH TO PASS THROUGH A 200-MESH SCREEN, AND (D) POLYMERIZING THE GROUND POLYESTER AT 200* TO 235* C. IN A FLUIDIZED-BED SOLID-PHASE POLYMERIZATION TO AN INTRINSIC VISCOSITY OF AT LEAST 0.8. 